Phase locked loops, or PLLs for short, are frequently used in order to produce signals at a stable frequency. For example, phase locked loops are used in transmitting arrangements for stationary or mobile communication purposes. In some transmission methods, a transmission signal is frequency-modulated. In this case, a phase locked loop can be used to produce a frequency which is stable in places.
Conventional phase locked loops have a phase detector, a charge pump and an oscillator in the forward path. One output of the oscillator is connected to one input of the phase detector in order to form a backward path.
Once a phased locked loop has settled in a state with a stable frequency after being switched on, it is referred to as being locked.
Phase locked loops exist, which are referred to as Type I phase locked loops, and have a loop filter with a non-integrating transfer function. Other phase locked loops are referred to as Type II phase locked loops, and have a loop filter with an integrating characteristic. In the case of Type I phase locked loops, monitoring is normally carried out to determine whether the duty ratio of the charge pump is tending to zero, in order to identify the locked state. A time period in which the charge pump is active, divided by a period duration, can be referred to as the duty ratio. Circuits such as these cannot be used for Type II phase locked loops, since the duty ratio of the charge pump in general differs from zero in this case, even in the locked state.
The aim is for a circuit arrangement to be suitable for detection of a locking condition for a phase locked loop of both the integrating type and the non-integrating type. The aim is for one method to allow determination of a locked state of a phase locked loop.